1. Field of the Invention
The invention is in the field of electronic design automation (EDA), and more particularly, is related to receiver models for that enable accurate timing analyses.
2. Related Art
An electronic design automation (EDA) system is a computer software system used for designing integrated circuit (IC) devices. The EDA system typically receives one or more high level behavioral descriptions of an IC device (e.g., in HDL languages like VHDL, Verilog, etc.) and translates (“synthesizes”) this high-level design language description into netlists of various levels of abstraction. A netlist describes the IC design and is composed of nodes (functional elements) and edges, e.g., connections between nodes. At a higher level of abstraction, a generic netlist is typically produced based on technology independent primitives.
The generic netlist can be translated into a lower level technology-specific netlist based on a technology-specific (characterized) cell library that has gate-specific models for each cell (i.e., a functional element, such as an AND gate, an inverter, or a multiplexer). The models define performance parameters for the cells; e.g., parameters related to the operational behavior of the cells, such as power consumption, delay, and noise. The netlist and cell library are typically stored in computer readable media within the EDA system and are processed and verified using many well-known techniques.
FIG. 1 shows a simplified representation of an exemplary digital ASIC design flow. At a high level, the process starts with the product idea (step E100) and is realized in an EDA software design process (step E110). When the design is finalized, it can be taped-out (event E140). After tape out, the fabrication process (step E150) and packaging and assembly processes (step E160) occur resulting, ultimately, in finished chips (result E170).
The EDA software design process (step E110) is actually composed of a number of steps E112-E130, shown in linear fashion for simplicity. In an actual ASIC design process, the particular design might have to go back through steps until certain tests are passed. Similarly, in any actual design process, these steps may occur in different orders and combinations. This description is therefore provided by way of context and general explanation rather than as a specific, or recommended, design flow for a particular ASIC.
A brief description of the components steps of the EDA software design process (step E110) will now be provided. During system design (step E112), the designers describe the functionality that they want to implement and can perform what-if planning to refine functionality, check costs, etc. Hardware-software architecture partitioning can occur at this stage. Exemplary EDA software products from Synopsys, Inc. that can be used at this step include Model Architect, Saber, System Studio, and DesignWare® products.
During logic design and functional verification (step E114), the VHDL or Verilog code for modules in the system is written and the design is checked for functional accuracy. More specifically, does the design as checked to ensure that produces the correct outputs. Exemplary EDA software products from Synopsys, Inc. that can be used at this step include VCS, VERA, DesignWare®, Magellan, Formality, ESP and LEDA products.
During synthesis and design for test (step E116), the VHDL/Verilog is translated to a netlist. The netlist can be optimized for the target technology. Additionally, the design and implementation of tests to permit checking of the finished chip occurs. Exemplary EDA software products from Synopsys, Inc. that can be used at this step include Design Compiler®, Physical Compiler, Test Compiler, Power Compiler, FPGA Compiler, Tetramax, and DesignWare® products.
During design planning (step E118), an overall floorplan for the chip is constructed and analyzed for timing and top-level routing. Exemplary EDA software products from Synopsys, Inc. that can be used at this step include Jupiter and Floorplan Compiler products.
During netlist verification (step E120), the netlist is checked for compliance with timing constraints and for correspondence with the VHDL/Verilog source code. Exemplary EDA software products from Synopsys, Inc. that can be used at this step include VCS, VERA, Formality and PrimeTime products.
During physical implementation (step E122), placement (positioning of circuit elements) and routing (connection of the same) is performed. Exemplary EDA software products from Synopsys, Inc. that can be used at this step include the Astro product.
During analysis and extraction (step E124), the circuit function is verified at a transistor level, this in turn permits what-if refinement. Exemplary EDA software products from Synopsys, Inc. that can be used at this step include Star RC/XT, Raphael, and Aurora products.
During physical verification (step E126), various checking functions are performed to ensure correctness for: manufacturing, electrical issues, lithographic issues, and circuitry. Exemplary EDA software products from Synopsys, Inc. that can be used at this step include the Hercules product.
During resolution enhancement (step E128), geometric manipulations of the layout are performed to improve manufacturability of the design. Exemplary EDA software products from Synopsys, Inc. that can be used at this step include the iN-Phase, Proteus, and AFGen products.
Finally, during mask data preparation (step E130), the “tape-out” data for production of masks for lithographic use to produce finished chips is performed. Exemplary EDA software products from Synopsys, Inc. that can be used at this step include the CATS(R) family of products.
As indicated in FIG. 1, timing analyses can be performed at various points along the EDA process, such as during synthesis, design planning, netlist verification, and analysis (as indicated by the bolded chevrons). The accuracy of these timing analyses is critical to the quality of final IC produced using EDA systems. A timing analysis is performed at the transistor level, and makes use of the performance data included in the characterized cell library. To perform a timing analysis, the IC design (or a portion of the IC) is modeled as a network of drivers and receivers. Cells designated as drivers provide stimuli to the network, and the resulting waveforms are received by the cells designated as receivers.
For example, FIG. 2A shows a schematic diagram of a sample driver-receiver network 200 that includes a driver 210 and a receiver 230. An input pin 211 of driver 210 receives a driver input signal S_IND and generates a driver output signal S_OUTD at a driver output pin 212. This signal is transmitted across an interconnect element 220 and is received as a receiver input signal S_INR at a receiver input pin 231 of receiver 230 (depicted as an inverter for exemplary purposes). Receiver 230 processes receiver input signal S_INR and generates a receiver output signal S_OUTR at a receiver output pin 232. Note that receiver 230 can also function as a driver for downstream cells, as indicated by load 240 connected to receiver output pin 232.
Because signals do not propagate instantly through a real-world circuit (e.g., due to propagation delays and parasitics), signals S_IND, S_OUTD, S_INR, and S_OUTR will have differing slew and delay characteristics. In the context of a timing analysis, “slew” represents the time required for a signal to transition between an upper threshold voltage and a lower threshold voltage (or vice versa), while “delay” represents the time required for the signal to transition from either the upper or lower rail (supply voltage) to a gate threshold voltage. Meanwhile, the gate threshold voltage typically represents the voltage at which the transistor switches state (from off to on, or vice versa).
The concepts of slew and delay are depicted in FIG. 2B, which shows a graph of a sample signal S_SAMP that represents a general signal provided to, or generated by, a cell within an IC design. Signal S_SAMP transitions between a lower rail voltage RL and an upper rail voltage RU, which represent the operating (supply) voltages for the cell. To define the slew and delay characteristics of signal S_SAMP, an upper threshold voltage THU, a lower threshold voltage THL, and a gate threshold voltage THG are selected. Upper threshold voltage THU, lower threshold voltage THL, and gate threshold voltage THG are typically selected to be predetermined percentages of the difference between rail voltages RU and RL. For example, lower threshold THL and upper threshold THU could be selected to be 20% and 80%, respectively, of the difference between upper rail voltage RU and lower rail voltage RL. Likewise, gate threshold voltage THG could be selected to be midway (i.e., 50% of the difference) between upper rail voltage RU and lower rail voltage RL.
Once lower threshold voltage THL, gate threshold voltage THG, and upper threshold voltage THV have been defined, delay and slew values can be determined for signal S_SAMP. For example, signal S_SAMP reaches gate threshold voltage THG at a time T2. Therefore, the delay value for signal S_SAMP is equal to the difference between times T2 and T0 (i.e., T2−T0). Similarly, since signal S_SAMP reaches lower threshold voltage THL and upper threshold voltage THU at times T1 and T3, respectively, the slew value for signal S_SAMP is the difference between times T3 and T1 (i.e., T3−T1). Delay and slew values can be determined in a similar manner for a signal transitioning from upper rail voltage RU to lower rail voltage RL.
In an EDA system, a characterized cell library is generated by fitting mathematical models to actual delay data (i.e., measured data or simulated (SPICE) data). Typically, a CMOS cell operating as a receiver is modeled as a single capacitor. For example, FIG. 3A shows receiver cell 230 of FIG. 2A replaced with a conventional receiver model 230-ST that includes a resistor R_ST and a capacitor C_ST serially coupled between receiver input pin 231 and ground. The value of capacitor C_ST is selected such that a model receiver input signal S_INR-ST generated by the RC circuit in response to driver output signal S_OUTD fits the actual (measured or simulated) receiver input signal S_INR. Typically, a different capacitance value is determined for rising signals, falling signals, “best case” (fastest) transitions, and “worst case” (slowest) transitions. Furthermore, since cell performance generally changes with operating conditions such as temperature and voltage, a new set of capacitance values are often generated across a range of operating conditions.
Thus, a receiver model entry in a conventional characterized cell library generally includes a set of capacitance values, with each single capacitance value being referenced by a signal type (rise, fall, best case, worst case) and set of operating conditions. For example, FIG. 3B shows a characterized library cell entry 300 for a receiver model of cell 230 (shown in FIG. 3A). Cell entry 300 includes a set of capacitance values C_ST referenced by signal type and operating conditions. For example, for a rising signal generated under operating conditions OP1, cell 230 is modeled (as a receiver) using a capacitance value C_ST(R1). Likewise, for a falling signal under the same operating conditions, cell 230 is modeled using a capacitance value C_ST(F1), while a rising signal under a set of operating conditions OP2 leads to the selection of a capacitance value C_ST(R2) to model cell 230.
Unfortunately, as device sizes continue to shrink, the behavior of a signal at a receiver can no longer be modeled by a single (static) capacitance value, as dynamic effects (e.g., the Miller Effect) begin to affect the signal shape. For example, FIG. 3C shows an actual signal S_INR-A1 (indicated by the bold curve) measured at the input pin of a receiver cell instantiated using 0.12 μm technology. Also depicted are model signals S_INR-ST1, S_INR-ST2, and S_INR-ST3 (indicated by the dashed curves), each having been generated using the above-described single-capacitance receiver model (with each of the signals being generated using a different capacitance value).
Because the curvature of signal S_INR-A1 varies significantly over the course of the signal transition, none of model signals S_INR-ST1, S_INR-ST2, and S_INR-ST3 can accurately model both the delay and slew characteristics of actual signal S_INR-A1. For example, model signal S_INR-ST1 provides a relatively good match to the actual delay of actual signal S_INR-A1. However, because model signal S_INR-ST1 reaches the upper threshold voltage THU much sooner than does actual signal S_INR-A1, the slew value generated by model signal S_INR-ST1 is much lower than the actual slew value of signal S_INR-A1. Unfortunately, while the model capacitance can be adjusted to reduce the slew error, such as in model signals S_INR-ST2 and S_INR-ST3, any such adjustment increases the model delay error.
Thus, conventional cell receiver models can be inadequate for the timing analysis of modern IC designs. Accordingly, it is desirable to provide a cell receiver model that can accurately represent the delay and slew characteristics of a CMOS cell.